RF plasmas are used in the manufacture of devices such as integrated circuits, micromechanical devices, flat panel displays, and other devices. RF plasma sources used in modern plasma etch applications are required to provide a high plasma uniformity and a plurality of plasma controls, including independent plasma profile, plasma density, and ion energy controls. RF plasma sources typically must be able to sustain a stable plasma in a variety of process gases and under a variety of different conditions (e.g. gas flow, gas pressure, etc.). In addition, it is desirable that RF plasma sources produce a minimum impact on the environment by operating with reduced energy demands and reduced EM emission.
Various plasma sources are known for achieving these stringent plasma process requirements. Multi-frequency capacitively coupled plasma (CCP) sources have been used for independent control of ion energy and plasma density. CCP plasma sources, however, have some intrinsic problems and limitations. For instance: (a) gas pressure ranges are typically limited to low pressures; (b) high-density plasma generation requires very high frequency RF, causing problems with plasma uniformity, emissions, etc.; (c) there is interference between higher and lower frequency RF sheaths; (d) the wafer edge area is prone to severe nonuniformity; and (e) a CCP source has a narrow process window. Accordingly, CCP sources are not always suitable for certain plasma process operations.
Inductively coupled plasma (ICP) sources combined with RF bias have also been used, for example, to provide independent control of ion energy and plasma density. ICP sources can easily produce high-density plasma using standard 13.56 MHz and lower frequency RF power generators. Indeed, it is known to use multi-coil ICP sources to provide good plasma control and high plasma density. For instance, in one known ICP source, two coils are placed on top of a dielectric window separating plasma from the air. The two coils are powered with an RF generator and the power distribution function between the coils is assigned to a matcher. This arrangement can be very complex and expensive. In addition, the communication between coils above the dielectric window and in the plasma makes it difficult to provide true independent control of power distribution into the plasma. This design also limits the range of power distribution between coils such that the central coil still receives power when power to the central coil is not needed, limiting the operational range of the tool.
A known multi-coil ICP source is disclosed in U.S. Pat. No. 6,267,074. This ICP source uses three separated coils, three power generators, multiple gas injectors and provides a complete control over plasma. The ICP source has, however, three generators, three matchers and an extremely expensive dielectric window with very complex shape and multiple channels for gas injection. The capital cost and maintenance cost of such a system is not justified for most etch processes.
Another common problem with ICP sources is a severe sputtering of a dielectric plate separating an ICP coil from a process chamber due to RF power capacitive coupling from the coil to plasma and very high voltage (a few kV per turn) applied to the coil. The sputtering both affects plasma and increases the capital cost of the tool and its maintenance cost. Overall process controllability and, finally, process yield deteriorates.
Yet another common problem with ICP systems is an azimuthal nonuniformity caused by the capacitive coupling of the coil. Such azimuthal nonuniformity can be caused for different reasons. One reason, for example, is that for secondary electrons emitted from the surface, the sheath is collisionless. These electrons enter the plasma with energy strongly dependent on the position from where the electrons were emitted. Electrons that were emitted near the ends of a coil have significantly higher energy than those emitted near the center of a coil or away from the coil. Although these electrons quickly mix in the volume, they do create noticeable azimuthal plasma nonuniformity.
To eliminate both sputtering and azimuthal nonuniformity caused by a capacitive coupling of a coil, one can use a Faraday shield as disclosed in U.S. Pat. Nos. 7,232,767, 6,551,447, and U.S. Patent Application Publication No. 2007/0181257. A Faraday shield also makes matching the coil to the power generator easier, more stable and less prone to plasma conditions. However, since a well-designed Faraday shield absorbs the capacitive component of the RF, the RF power transfer to the plasma is reduced. Further, since it is the capacitive component of the RF that initiates the plasma, a well-designed Faraday shield usually requires additional means for discharge ignition.
Some Faraday shield designs can also improve the radial plasma profile for many etch processes without using an additional coil. In particular, for many processes the bulk etch rate is center-fast, even if one uses only a single coil near the edge of the wafer. For instance, one exemplary known Faraday shield design selectively blocks any power coupling in the center of the source to correct for an intrinsic center-fast etch profile. However, this method of controlling the etch profile is inflexible in that the Faraday shield has to be redesigned for specific process chemistries, depending on the inherent etch profile of that chemistry. By adding a second coil, the etch profile can be adjusted dynamically, without changing the hardware, providing greatly increased process flexibility.
The use of a second coil with a Faraday shield in the center of a plasma source poses its own difficulties. Because of high RF voltage and requirements for safe spacing between parts, providing a second coil that is truly independent of a primary coil is a difficult task. One also has to provide means for the synchronization of generators (if using different generators) to prevent the coils from working against each other, further adding to the cost of the system.
The root cause of many problems in ICP sources is that every coil in any ICP source has the same function and works together in a similar way as other coils, so that the only differences between the coils is their respective designated areas of the wafer. Thus, a need exists for a multi-coil ICP source that avoids the above-mentioned problems and disadvantages. An ICP source that includes at least one secondary coil that can have a different structure from the primary coil and is operable to perform a different function from the primary coil would be particularly useful.